Power conversion circuit and power conversion apparatus with same

ABSTRACT

A power conversion circuit includes a first terminal, a second terminal, a first switching conversion unit, a second switching conversion unit, a flying capacitor and a magnetic element. The first switching conversion unit includes a first switch and a third switch. The second switching conversion unit includes a second switch and a fourth switch. The magnetic element includes two first windings and a second winding. A first one of the two first windings is serially connected between the flying capacitor and the second terminal. A second one of the two first windings is serially connected between the second switch and the second terminal. The second winding is serially connected with the flying capacitor and the first one of the two first windings. A turn ratio between the second winding, the first one of the two first windings and the second one of the two first windings is N:1:1.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation application of U.S. patentapplication Ser. No. 17/115,605 filed on Dec. 8, 2020 and entitled“POWER CONVERSION CIRCUIT AND POWER CONVERSION APPARATUS WITH SAME”,which claims priority to China Patent Application No. 201911268267.8,filed on Dec. 11, 2019. The entire contents of the above-mentionedpatent application are incorporated herein by reference for allpurposes.

FIELD OF THE INVENTION

The present invention relates to a power conversion circuit and a powerconversion apparatus, and more particularly to a power conversioncircuit with an adjustable voltage gain and a power conversionapparatus.

BACKGROUND OF THE INVENTION

Nowadays, the resonant power conversion circuit having a non-isolatedcircuit topology with extended duty cycle is widely used in theapplication of high current. The resonant power conversion circuits areusually divided into a symmetrical type and an asymmetrical type. In theprior power conversion circuit, regardless of the type, the voltage gainis set to be fixed, which means the ratio of the output voltage to theinput voltage is fixed. That is, the voltage gain cannot be determinedand adjusted according to the practical requirements. However, the fixedvoltage gain may limit the applications of the resonant power conversioncircuit.

SUMMARY OF THE INVENTION

An object of the present invention provides a power conversion circuitwith an adjustable voltage gain. Since the voltage gain is adjustable,the applications of the power conversion circuit are expanded.

Another object of the present invention provides a power conversionapparatus with a power conversion circuit.

In accordance with an aspect of the present invention, a powerconversion circuit is provided. The power conversion circuit includes afirst terminal, a second terminal, a first switching conversion unit, asecond switching conversion unit, a flying capacitor and a magneticelement. The first terminal includes a first positive electrode and afirst negative electrode. The second terminal includes a second positiveelectrode and a second negative electrode. The second negative electrodeis electrically connected with the first negative electrode. The firstswitching conversion unit includes a first switch and a third switch,which are electrically connected with each other in series. The secondswitching conversion unit includes a second switch and a fourth switch,which are electrically connected with each other in series. A firstterminal of the first switch is electrically connected with a firstterminal of the second switch. A second terminal of the first switch iselectrically connected with the first positive electrode. The thirdswitch is serially connected with the first switch. The fourth switch isserially connected with the second switch. A first terminal of the thirdswitch and a first terminal of the fourth switch are electricallyconnected with the first negative electrode. A second terminal of thefourth switch is electrically connected with a second terminal of thesecond switch. The first switch, the second switch, the third switch andthe fourth switch are periodically operated at a switching cycle. Theswitching cycle has a duty cycle. The magnetic element includes twofirst windings and a second winding. The two first windings and thesecond winding interact with each other to result in an electromagneticcoupling effect. The second terminals of the two first windings areopposite-polarity terminals and electrically connected with the secondpositive electrode. A first terminal of a first one of the two firstwindings is electrically connected with a second terminal of the thirdswitch. A first terminal of a second one of the two first windings iselectrically connected with the second terminal of the fourth switch andthe second terminal of the second switch. The second winding and theflying capacitor are serially connected between the first terminal ofthe first switch and the first terminal of the first one of the twofirst windings. Moreover, a turn ratio between the second winding, thefirst one of the two first windings and the second one of the two firstwindings is N:1:1, and N is a positive value. The switching cyclecomprises a first working period and a second working period. A currentflowing through the second winding is equal to a current flowing throughthe first one of the two first windings during the first working period.The current flowing through the second winding is equal to a currentflowing through the second one of the two first windings during thesecond working period.

In accordance with another aspect of the present invention, powerconversion apparatus is provided. The power conversion apparatusincludes M power conversion circuits. Each power conversion circuit hasthe above circuitry structure. The first terminals of the M powerconversion circuits are electrically connected with each other. Thesecond terminals of the M power conversion circuits are electricallyconnected with each other.

In accordance with another aspect of the present invention, a powerconversion circuit is provided. The power conversion circuit includes afirst terminal, a second terminal, a first flying capacitor, a secondflying capacitor, a first switching conversion unit, a second switchingconversion unit and a magnetic element. The first terminal includes afirst positive electrode and a first negative electrode. The secondterminal includes a second positive electrode and a second negativeelectrode. The second negative electrode is electrically connected withthe first negative electrode. The first switching conversion unitincludes a first switch, a second switch and a third switch. A firstterminal of the first switch is electrically connected with the firstpositive electrode. A second terminal of the second switch iselectrically connected with a first terminal of the third switch. Asecond terminal of the third switch is electrically connected with thesecond negative electrode. The second switching conversion unit includesa fourth switch, a fifth switch and a sixth switch. A first terminal ofthe fourth switch is electrically connected with the first positiveelectrode. A second terminal of the fifth switch is electricallyconnected with a first terminal of the sixth switch. A second terminalof the sixth switch is electrically connected with the second negativeelectrode. A second terminal of the fourth switch is electricallyconnected with a first terminal of the second switch. A first terminalof the fifth switch is electrically connected with the second terminalof the first switch. The first switch, the second switch, the thirdswitch, the fourth switch, the fifth switch and the sixth switch areperiodically operated at a switching cycle. The switching cycle has aduty cycle. The magnetic element includes two first windings and twosecond windings. The two first windings and the two second windingsinteract with each other to result in an electromagnetic couplingeffect. The second terminals of the two first windings areopposite-polarity terminals and electrically connected with the secondpositive electrode. A first terminal of a first one of the two firstwindings is electrically connected with the second terminal of the fifthswitch and the first terminal of the sixth switch. A first terminal of asecond one of the two first winding is electrically connected with thesecond terminal of the second switch and the first terminal of the thirdswitch. A first one of the two second windings and the second flyingcapacitor are serially connected between the second terminal of thefourth switch and the second terminal of the fifth switch. A second oneof the two second windings and the first flying capacitor are seriallyconnected between the second terminal of the first switch and the secondterminal of the second switch. Moreover, a turn ratio between the firstone of the two second windings, the second one of the two secondwindings, the first one of the two first windings and the second one ofthe two first windings is N:N:1:1, and N is a positive value. Theswitching cycle comprises a first working period and a second workingperiod. A total current flowing through the two second windings is equalto a current flowing through the first one of the two first windingsduring the first working period. The total current flowing through thetwo second windings is equal to a current flowing through the second oneof the two first windings during the second working period.

In accordance with another aspect of the present invention, powerconversion apparatus is provided. The power conversion apparatusincludes M power conversion circuits. Each power conversion circuit hasthe above circuitry structure. The first terminals of the M powerconversion circuits are electrically connected with each other. Thesecond terminals of the M power conversion circuits are electricallyconnected with each other.

The above contents of the present invention will become more readilyapparent to those ordinarily skilled in the art after reviewing thefollowing detailed description and accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic circuit diagram illustrating a power conversioncircuit according to a first embodiment of the present invention;

FIG. 1B is a schematic timing waveform diagram illustrating the on/offstates of associated switches in the power conversion circuit as shownin FIG. 1A and associated voltage signals and current signals;

FIG. 1C is a schematic equivalent circuit diagram of the powerconversion circuit as shown in FIG. 1A in the time interval between thetime point t0 and the time point t1 as shown in FIG. 1B;

FIG. 1D is a schematic equivalent circuit diagram of the powerconversion circuit as shown in FIG. 1A in the time interval between thetime point t2 and the time point t3 as shown in FIG. 1B;

FIG. 1E is a schematic circuit diagram illustrating a power conversioncircuit according to a second embodiment of the present invention;

FIG. 2A is a schematic circuit diagram illustrating a power conversioncircuit according to a third embodiment of the present invention;

FIG. 2B is a schematic timing waveform diagram illustrating the on/offstates of associated switches in the power conversion circuit as shownin FIG. 2A and associated voltage signals and current signals;

FIG. 2C is a schematic equivalent circuit diagram of the powerconversion circuit as shown in FIG. 2A in the time interval between thetime point t0 and the time point t1 as shown in FIG. 2B;

FIG. 2D is a schematic equivalent circuit diagram of the powerconversion circuit as shown in FIG. 2A in the time interval between thetime point t2 and the time point t3 as shown in FIG. 2B;

FIG. 2E is a schematic circuit diagram illustrating a power conversioncircuit according to a fourth embodiment of the present invention;

FIG. 3 is a schematic circuit diagram illustrating a power conversionapparatus according to a first embodiment of the present invention; and

FIG. 4 is a schematic circuit diagram illustrating a power conversionapparatus according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described more specifically withreference to the following embodiments. It is to be noted that thefollowing descriptions of preferred embodiments of this invention arepresented herein for purpose of illustration and description only. It isnot intended to be exhaustive or to be limited to the precise formdisclosed.

FIG. 1A is a schematic circuit diagram illustrating a power conversioncircuit according to a first embodiment of the present invention. FIG.1B is a schematic timing waveform diagram illustrating the on/off statesof associated switches in the power conversion circuit as shown in FIG.1A and associated voltage signals and current signals. FIG. 1C is aschematic equivalent circuit diagram of the power conversion circuit asshown in FIG. 1A in the time interval between the time point t0 and thetime point t1 as shown in FIG. 1B. FIG. 1D is a schematic equivalentcircuit diagram of the power conversion circuit as shown in FIG. 1A inthe time interval between the time point t2 and the time point t3 asshown in FIG. 1B.

The power conversion circuit of the present invention has the functionof converting the electric power in a bidirectional manner. In case thata first terminal of the power conversion circuit is an input terminal, asecond terminal of the power conversion circuit is an output terminal.In case that the first terminal of the power conversion circuit is theoutput terminal, the second terminal of the power conversion circuit isthe input terminal. Moreover, the power conversion circuit is a resonantpower conversion circuit with extended duty cycle.

As shown in FIG. 1A, the circuit topology of the power conversioncircuit has an asymmetrical configuration. The power conversion circuit1 includes a first terminal (including a first positive electrode V1+and a first negative electrode V1−), a second terminal (including asecond positive electrode V2+ and a second negative electrode V2−), afirst switching conversion unit, a second switching conversion unit, afirst capacitor C1, a second capacitor C2, a flying capacitor Cb11 and amagnetic element T-1. The first negative electrode V1− and the secondnegative electrode V2− are connected to a ground terminal. The firstswitching conversion unit includes a first switch S11 and a third switchSr11, which are electrically connected in series. The second switchingconversion unit includes a second switch S12 and a fourth switch Sr12,which are electrically connected in series. The first switch S11, thesecond switch S12, the third switch Sr11 and the fourth switch Sr12 areperiodically operated at a switching cycle Ts. The switching cycle has aduty cycle.

The first terminal of the first switch S11 is electrically connectedwith the first terminal of the second switch S12. The second terminal ofthe first switch S11 is electrically connected with the first positiveelectrode V1+. The second terminal of the fourth switch Sr12 iselectrically connected with the second terminal of the second switchS12. The first terminal of the third switch Sr11 and the first terminalof the fourth switch Sr12 are connected with the first negativeelectrode V1−. The on/off states of the first switch S11 and the fourthswitch Sr12 are identical. The on/off states of the second switch S12and the third switch Sr11 are identical. The phase difference between acontrol signal of the first switch S11 and a control signal of thesecond switch S12 is 180 degrees. The ON time durations of the firstswitch S11 and the second switch S12 are less than or equal to 0.5×Tsand greater than or equal to 0.4×Ts. The first capacitor C1 iselectrically connected between the first positive electrode V1+ and thefirst negative electrode V1−. The second capacitor C2 is electricallyconnected between the second positive electrode V2+ and the secondnegative electrode V2−.

The magnetic element T-1 includes two first windings T11, T12 and asecond winding T13. These windings are wound around the same pillar of amagnetic core of the magnetic element to result in an electromagneticcoupling effect. The second terminals of the two first windings T11 andT12 are electrically connected with the second positive electrode V2+.The second terminals of the two first windings T11 and T12 areopposite-polarity terminals. The first terminal of the first winding T11is electrically connected with the second terminal of the third switchSr11. The first terminal of the first winding T12 is electricallyconnected with the second terminal of the fourth switch Sr12 and thesecond terminal of the second switch S12. The second winding T13 and theflying capacitor Cb11 are connected with each other in series to form aserially-connected branch. The first end of the serially-connectedbranch is connected with the first terminal of the first switch S11. Thesecond end of the serially-connected branch is connected with the secondterminal of the third switch Sr11 and the first terminal of the firstwinding T11. The turn ratio between the second winding T13, the firstwinding T11 and the first winding T12 is N:1:1, wherein N is a positivevalue, and preferably a positive integer.

In the serially-connected branch, the positions and sequence of thesecond winding T13 and the flying capacitor Cb11 are not restricted. Inan embodiment, the first terminal of the second winding T13 iselectrically connected with the first terminal of the first winding T11.The first terminal of the second winding T13 and the first terminal ofthe first winding T11 are opposite-polarity terminals. The secondterminal of the second winding T13 is electrically connected with theflying capacitor Cb11. In another embodiment, a terminal of the flyingcapacitor Cb11 is electrically connected with the first terminal of thefirst winding T11, and the other terminal of the flying capacitor Cb11is electrically connected with the first terminal of the second windingT13. The first terminal of the second winding T13 and the first terminalof the first winding T11 are opposite-polarity terminals.

The working principle of the power conversion circuit 1 will bedescribed as follows. For illustration, taking the first terminal of thepower conversion circuit 1 as the input terminal, and the secondterminal of the power conversion circuit 1 as the output terminal forexample.

Please refer to FIGS. 1B, 1C and 1D again. When the power conversioncircuit 1 is in a steady state, the time interval between the time pointt0 and the time point t4 is equal to the switching cycle Ts.

In the time interval between the time point t0 and the time point t1,the first switch S11 and the fourth switch Sr12 are in the on state.This time interval is also referred as a first working period. Theflying capacitor Cb11 is charged by the input voltage V1 through thefirst switch S11. The electric energy is transmitted from the inputterminal to the output terminal through the second winding T13 and thefirst winding T11. The first winding T12 is in a freewheeling statethrough the fourth switch Sr12. Meanwhile, the current flowing throughthe second winding T13 is equal to the current flowing through the firstwinding T11. The equivalent circuit diagram is shown in FIG. 1C. In FIG.1C, T11′, T12′ and T13′ are the ideal windings corresponding to thewindings T11, T12 and T13, Lr11, Lr12 and Lr13 are equivalent leakageinductors corresponding to the windings, and Lm1 is an equivalentmagnetized inductor of the magnetic element T-1. Due to the resonanteffect between the equivalent resonant inductor of the power conversioncircuit 1 (i.e., the equivalent resonant inductor resulted by theequivalent leakage inductors Lr11, Lr12 and Lr13) and the flyingcapacitor Cb11, the resonant currents iLr11 and iLr12 are generated. Theequivalent magnetized current generated by the magnetic element T-1 isiLm1.

The associated voltages of the power conversion circuit 1 can be seen inFIG. 1C. The voltage between the two terminals of the ideal firstwinding T12′ is equal to the voltage V2 of the second terminal of thepower conversion circuit 1. As mentioned above, the turn ratio betweenthe second winding T13, the first winding T11 and the first winding T12is N:1:1. Consequently, the voltage between the two terminals of theideal first winding T11′ is also equal to the voltage V2, and thevoltage between the two terminals of the ideal second winding T13′ isequal to N×V2.

Consequently, the voltage V1 of the first terminal of the powerconversion circuit 1 may be expressed by the following mathematicalformula:V1=Vc11+(2+N)×V2  (1)

In the above mathematic formula, Vc11 is the terminal voltage of theflying capacitor Cb11.

At the time point t1, the resonant currents iLr11 and iLr12 are equal tothe magnetized currents iLm1 and −iLm1, respectively. Meanwhile, thefirst switch S11 and the fourth switch Sr12 are turned off. Since thezero current switching (ZCS) function is achieved, the switching loss isdecreased and the energy transfer efficiency of the power conversioncircuit 1 is enhanced.

In the time interval between the time point t1 and the time point t2,all switches are turned off. The magnetized current iLm1 flowing throughthe magnetic element T-1 is in the freewheeling state. In addition, thecharges on the parasitic capacitors of the second switch S12 and thethird switch Sr11 are extracted. Consequently, the terminal voltages ofthe second switch S12 and the third switch Sr11 are decreased. In anembodiment, the second switch S12 and the third switch Sr11 are turnedon when the terminal voltages of the second switch S12 and the thirdswitch Sr11 are decreased to 50% of the respective initial voltages(i.e., the terminal voltages at the time point t1). Consequently, theswitching loss is decreased, and the energy transfer efficiency and thepower density of the power conversion circuit 1 are enhanced.

In another embodiment, the inductance of the magnetic element T-1 iscontrolled. Consequently, the inductance of the equivalent magnetizedinductor Lm1 of the magnetic element T-1 is low enough, and themagnetized current iLm1 flowing through the equivalent magnetizedinductor Lm1 is high enough. Since the charges on the parasiticcapacitors of the second switch S12 and the third switch Sr11 areextracted completely, the terminal voltages of the second switch S12 andthe third switch Sr11 are decreased to zero. At this time, the secondswitch S12 and the third switch Sr11 are turned on. Consequently, thezero voltage switching (ZVS) function is achieved. In such way, theswitching loss is further decreased, and the energy transfer efficiencyand the power density of the power conversion circuit 1 are furtherenhanced.

In the time interval between the time point t2 and the time point t3,the second switch S12 and the third switch Sr11 are in the on state.This time interval is also referred as a second working period. Theenergy stored in the flying capacitor Cb11 is transmitted to the outputterminal through the second switch S12, the first winding T12, the thirdswitch Sr11 and the second winding T13. The first winding T11 is in thefreewheeling state through the third switch Sr11. Meanwhile, the currentflowing through the second winding T13 is equal to the current flowingthrough the first winding T12. The equivalent circuit diagram is shownin FIG. 1D. Due to the resonant effect between the of the powerconversion circuit 1 (i.e., the equivalent resonant inductor resulted bythe leakage inductors Lr11, Lr12 and Lr13) and the flying capacitorCb11, the resonant currents iLr11 and iLr12 are generated. Theequivalent magnetized current generated by the magnetic element T-1 isiLm1.

The associated voltages of the power conversion circuit 1 can be seen inFIG. 1D. The voltage between the two terminals of the ideal firstwinding T11′ is equal to the voltage V2 of the second terminal of thepower conversion circuit 1. As mentioned above, the turn ratio betweenthe second winding T13, the first winding T11 and the first winding T12is N:1:1. Consequently, the voltage between the two terminals of theideal first winding T12′ is also equal to the voltage V2, and thevoltage between the two terminals of the ideal second winding T13′ isequal to N×V2.

Consequently, the voltage Vc11 of the flying capacitor Cb11 may beexpressed by the following mathematical formula:Vc11=(2+N)×V2  (2)

The energy stored in the flying capacitor Cb11 in the time intervalbetween the time point t0 and the time point t1 is transmitted to theoutput terminal in the time interval between the time point t2 and thetime point t3. Consequently, after the formula (2) is introduced intothe formula (1), the voltage V1 of the first terminal of the powerconversion circuit 1 may be deduced as: V1=(4+2N)×V2.

At the time point t3, the resonant currents iLr11 and iLr12 are equal tothe magnetized currents iLm1 and −iLm1, respectively. Meanwhile, thesecond switch S12 and the third switch Sr11 are turned off. Since thezero current switching (ZCS) function is achieved, the switching loss isdecreased and the energy transfer efficiency of the power conversioncircuit 1 is enhanced.

In the time interval between the time point t3 and the time point t4,all switches are turned off. The magnetized current iLm1 flowing throughthe first windings T11 and T12 is in the freewheeling state. Inaddition, the charges on the parasitic capacitors of the first switchS11 and the fourth switch Sr12 are extracted. Consequently, the terminalvoltages of the first switch Si 1 and the fourth switch Sr12 aredecreased. In an embodiment, the first switch S11 and the fourth switchSr12 are turned on when the terminal voltages of the first switch S11and the fourth switch Sr12 are decreased to 50% of the respectiveinitial voltages (i.e., the terminal voltages at the time point t1).Consequently, the switching loss is decreased, and the energy transferefficiency and the power density of the power conversion circuit 1 areenhanced.

In another embodiment, the inductance of the magnetic element T-1 iscontrolled. Consequently, the inductance of the equivalent magnetizedinductor Lm1 of the magnetic element T-1 is low enough, and themagnetized current iLm1 flowing through the equivalent magnetizedinductor Lm1 is high enough. Since the charges on the parasiticcapacitors of the first switch S11 and the fourth switch Sr12 areextracted completely, the terminal voltages of the first switch S11 andthe fourth switch Sr12 are decreased to zero. At this time, the firstswitch S11 and the fourth switch Sr12 are turned on. Consequently, thezero voltage switching (ZVS) function is achieved. In such way, theswitching loss is further decreased, and the energy transfer efficiencyand the power density of the power conversion circuit 1 are furtherenhanced.

In the time interval between the time point t0 and the time point t1 andin the time interval between the time point t2 and the time point t3,the resonant current iLr11 flows through the first winding T11 and theresonant current iLr12 flows through the first winding T12. In addition,the frequency of each of the resonant current iLr11 and the resonantcurrent iLr12 is equal to the switching frequency. In this embodiment,the resonant cycle and the switching cycle are nearly equal.

In some other embodiments, the capacitance of the flying capacitor Cb11is larger, and the inductance of the equivalent resonant inductor issmaller. Consequently, if the resonant currents iLr11 and iLr12 arerespectively greater than the magnetized currents iLm1 and −iLm1 in thetime interval between the time point t0 and the time point t1, thecorresponding switches are turned off. If the resonant currents iLr11and iLr12 are respectively greater than −iLm1 and iLm1 in the timeinterval between the time point t2 and the time point t3, thecorresponding switches are turned off. The turn-off current is greaterthan zero. However, since the inductance of the equivalent resonantinductor is low, the power loss caused by the non-zero currentturning-off action may be neglected. In other words, the switching cycleof the power conversion circuit 1 is less than or equal to the resonantcycle of the resonant current. For reducing the power loss andincreasing the energy transfer efficiency, it is preferred that theswitching cycle Ts is greater than or equal to a half of the resonantcycle.

In an embodiment, the ratio of the input voltage V1 to the outputvoltage V2 of the power conversion circuit 1 is (4+2N):1. That is, theratio of the input voltage V1 to the output voltage V2 may be adjustedaccording to the change of N. In this embodiment, the magnetic elementT-1 includes the two first windings T11, T12 and the second winding T13.These windings interact with each other to result in the electromagneticcoupling effect. Moreover, the turn ratio between the second windingT13, the first winding T11 and the first winding T12 is N:1:1. Thesecond winding T13 is disposed on a specific position of the powerconversion circuit 1. Since the voltage gain of the power conversioncircuit 1 is adjustable according to the turn number of the secondwinding T13, the applications of the power conversion circuit 1 areexpanded.

In the embodiment, as shown in FIGS. 1C and 1D, the equivalent leakageinductors corresponding to the first windings T11, T12 and the secondwinding T13 are Lr11, Lr12 and Lr13, respectively. For clearly analyzingthe relationship between the resonant currents, the magnetized currentiLm1 and the magnetized voltage VLm1 of the equivalent magnetizedinductor Lm1 are neglected in the following example. In the timeinterval between the time point t0 and the time point t1 and the timeinterval between the time point t2 and the time point t3, the resonanteffect between the flying capacitor Cb11 and the equivalent leakageinductors Lr11, Lr12 and Lr13 is generated. In this embodiment, theresonant capacitor of the power conversion circuit 1 is the flyingcapacitor Cb11, and the equivalent resonant inductance is the sum of theinductances of the equivalent leakage inductors Lr11, Lr12 and Lr13. Ifthe magnetized current iLm1 is neglected, the output current io of thepower conversion circuit 1 may be expressed by the following mathematicformula:io=iLr11+iLr12  (3)

In the above mathematic formula, iLr11 is the resonant current flowingthrough the equivalent leakage inductor Lr11, and iLr12 is the resonantcurrent flowing through the equivalent leakage inductor Lr12.

In the time interval between the time point t0 and the time point t1,the resonant current iLr13 flowing through the equivalent leakageinductor Lr13 is equal to the resonant current iLr11 flowing through theequivalent leakage inductor Lr11. That is,iLr13=iLr11  (4)

According to the magnetic potential balance principle, the followingmathematic formula is obtained.N×iLr13+iLr11=iLr12  (5)

According to the above mathematic formulae (3), (4) and (5), theresonant current iLr12 is equal to (N+1)×io/(N+2), and the resonantcurrent iLr11 is equal to io/(N+2).

In the time interval between the time point t2 and the time point t3,the resonant current iLr13 flowing through the equivalent leakageinductor Lr13 is equal to the resonant current iLr12 flowing through theequivalent leakage inductor Lr12. That is,iLr13=iLr12  (6)

According to the magnetic potential balance principle, the followingmathematic formula is obtained.N×iLr13+iLr12=iLr11  (7)

According to the above mathematic formulae (3), (6) and (7), theresonant current iLr11 is equal to (N+1)×io/(N+2), and the resonantcurrent iLr12 is equal to io/(N+2).

From the above descriptions, the voltage gain of the power conversioncircuit 1 is adjustable according to the turn number of the secondwinding T13 of the magnetic element T-1. The sum of the resonantcurrents iLr11 and iLr12 in the time interval between the time point t0and the time point t1 and the sum of the resonant currents iLr11 andiLr12 in the time interval between the time point t2 and the time pointt3 are equal. In other words, the resonant effect of the powerconversion circuit 1 is not influenced by the second winding T13. Theterminal voltage Vc11 of the flying capacitor is obtained bysuperimposing a DC voltage with an AC resonant voltage. Typically, theDC voltage is equal to Vin/2, and thus the ratio of the DC voltage tothe input voltage is 0.5. When the device parameter distribution andother factors are taken into consideration, the ratio of the DC voltageto the input voltage (i.e., the terminal voltage of the first terminalof the power conversion circuit 1) is in the range between 0.4 and 0.6.The amplitude of the AC resonant voltage of the terminal voltage Vc11 isdetermined according to the inductance of the equivalent resonantinductor, the capacitance of the equivalent resonant capacitor, theswitching frequency of the power conversion circuit and the size of theload.

In the above embodiment, the equivalent resonant inductor comprises theleakage inductance caused by the coupling effect of the first windingsT11, T12 and the second winding T13 and the parasitic inductance of thewiring structure. When the resonant cycle, the switching cycle and thecapacitance of the flying capacitor Cb11 are taken into consideration,the coupling efficiency between every two of the first windings T11, T12and the second winding T13 is preferably greater than 0.9, but it is notlimited thereto.

FIG. 1E is a schematic circuit diagram illustrating a power conversioncircuit according to a second embodiment of the present invention. Incomparison with the first embodiment, the power conversion circuit ofthis embodiment further includes at least one external inductor (notshown). The position of the at least one external inductor may bedetermined according to the practical requirements. In an example, oneexternal inductor is serially connected between the second terminal ofthe first windings T11 or T12 and the second positive electrode V2+.That is, the external inductor is located at the position A. In anotherembodiment, two external inductors with the same inductance are seriallyconnected with two first windings T11 and T12. That is, the two externalinductors are located at the positions B1 or/and B2. In an example, atleast one external inductor is serially connected to the flyingcapacitor Cb11 and the second winding T13 in the serially-connectedbranch. For example, one external inductor is located at the positionC1, C2 or C3, two external inductors are respectively located at two ofthe positions C1, C2 and C3, or three external inductors arerespectively located at the positions C1, C2 and C3. Consequently, asuitable resonant cycle is acquired. Nevertheless, the at least oneexternal inductor is serially connected between the first terminal ofthe first switch S11 and the second positive electrode V2+.

In some embodiments, the third switch Sr11 and the fourth switch Sr12are replaced by diodes. The diodes are used as freewheeling diodes. Theswitching cycle of the power conversion circuit is less than or equal tothe resonant cycle. The equivalent circuit and the current waveform aresimilar to those of the above embodiment, and not redundantly describedherein. For example, the switches are controllable switches such as MOSswitches, SiC switches or GaN switches.

As mentioned above, the power conversion circuit 1 of the presentinvention has the function of converting the electric power in thebidirectional manner. Consequently, in case that the first terminal ofthe power conversion circuit 1 is the output terminal, the secondterminal of the power conversion circuit 1 is the input terminal. Theoperations are similar to those of the first embodiment, and are notredundantly described herein. Under this circumstance, the ratio of theinput voltage to the output voltage of the power conversion circuit 1 is1:(4+2N).

FIG. 2A is a schematic circuit diagram illustrating a power conversioncircuit according to a third embodiment of the present invention. FIG.2B is a schematic timing waveform diagram illustrating the on/off statesof associated switches in the power conversion circuit as shown in FIG.2A and associated voltage signals and current signals. FIG. 2C is aschematic equivalent circuit diagram of the power conversion circuit asshown in FIG. 2A in the time interval between the time point t0 and thetime point t1 as shown in FIG. 2B. FIG. 2D is a schematic equivalentcircuit diagram of the power conversion circuit as shown in FIG. 2A inthe time interval between the time point t2 and the time point t3 asshown in FIG. 2B. As shown in FIG. 2A, the circuit topology of the powerconversion circuit 2 has a symmetrical configuration. The powerconversion circuit 2 includes a first terminal (including a firstpositive electrode V1+ and a first negative electrode V1−), a secondterminal (including a second positive electrode V2+ and a secondnegative electrode V2−), a first capacitor C1, a second capacitor C2, afirst flying capacitor Cb21, a second flying capacitor Cb22, a firstswitching conversion unit, a second switching conversion unit and aresonant circuit T-2. The first negative electrode V1− and the secondnegative electrode V2− are connected to a ground terminal.

The first switching conversion unit includes a first switch S21, asecond switch S24 and a third switch Sr22. The second switchingconversion unit includes a fourth switch S23, a fifth switch S22 and asixth switch Sr21. The circuitry structure of the second switchingconversion unit is similar to the circuitry structure of the firstswitching conversion unit. The first terminal of the first switch S21 iselectrically connected with the first positive electrode V1+. The secondterminal of the first switch S21 is electrically connected with thefirst terminal of the fifth switch S22. The second terminal of the fifthswitch S22 is electrically connected with the first terminal of thesixth switch Sr21. The second terminal of the sixth switch Sr21 iselectrically connected with the second negative electrode V2−. The firstterminal of the fourth switch S23 is electrically connected with thefirst positive electrode V1+, and the fourth switch S23 and the firstswitch S21 are connected in parallel. The second terminal of the fourthswitch S23 is electrically connected with the first terminal of thesecond switch S24. The second terminal of the second switch S24 iselectrically connected with the first terminal of the third switch Sr22.The second terminal of the third switch Sr22 is electrically connectedwith the second negative electrode V2−. The first terminal of the firstflying capacitor Cb21 is electrically connected with the second terminalof the first switch S21 and the first terminal of the fifth switch S22.The second terminal of the first flying capacitor Cb21 is electricallyconnected with the second terminal of the second switch S24 and thefirst terminal of the third switch Sr22. The first terminal of thesecond flying capacitor Cb22 is electrically connected with the secondterminal of the fourth switch S23 and the first terminal of the secondswitch S24. The second terminal of the second flying capacitor Cb22 iselectrically connected with the second terminal of the fifth switch S22and the first terminal of the sixth switch Sr21. The first switch S21,the second switch S24, the third switch Sr22, the fourth switch S23, thefifth switch S22 and the sixth switch Sr21 are periodically operated ata switching cycle Ts. The switching cycle has a duty cycle.

The on/off states of the first switch S21, the second switch S24 and thesixth switch Sr21 are identical. The on/off states of the fourth switchS23, the fifth switch S22 and the third switch Sr22 are identical. Thephase difference between the control signal of the first switch S21 andthe control signal of the fourth switch S23 is 180 degrees. The timedurations of the first switch S21 and the fourth switch S23 are lessthan or equal to 0.5×Ts and greater than or equal to 0.4×Ts. The firstcapacitor C1 is electrically connected between the first positiveelectrode V1+ and the first negative electrode V1−. The second capacitorC2 is electrically connected between the second positive electrode V2+and the second negative electrode V2−.

The magnetic element T-2 includes two first windings T21, T22 and twosecond windings T23, T24. These windings are wound around the samepillar of a magnetic core of the magnetic element to result in anelectromagnetic coupling effect. The second terminals of the two firstwindings T21 and T22 are electrically connected with the second positiveelectrode V2+. The second terminals of the two first windings T21 andT22 are opposite-polarity terminals. The first terminal of the firstwinding T21 is electrically connected with the second terminal of thefifth switch S22 and the first terminal of the sixth switch Sr21. Thefirst terminal of the first winding T22 is electrically connected withthe second terminal of the second switch S24 and the first terminal ofthe third switch Sr22. The second winding T23 and the second flyingcapacitor Cb22 are connected with each other in series to form a firstserially-connected branch. The first serially-connected branch isconnected between the second terminal of the fourth switch S23 and thesecond terminal of the fifth switch S22. The second winding T24 and thefirst flying capacitor Cb21 are connected with each other in series toform a second serially-connected branch. The second serially-connectedbranch is connected between the second terminal of the first switch S21and the second terminal of the second switch S24. The turn ratio betweenthe second winding T23, the second winding T24, the first winding T21and the first winding T22 is N:N:1:1, wherein N is a positive value, andpreferably a positive integer.

In the first serially-connected branch, the positions and sequence ofthe second winding T23 and the second flying capacitor Cb22 are notrestricted. In an embodiment, the first terminal of the second windingT23 is electrically connected with the first terminal of the firstwinding T21. The first terminal of the second winding T23 and the firstterminal of the first winding T21 are opposite-polarity terminals. Thesecond terminal of the second winding T23 is electrically connected withthe second flying capacitor Cb22. In another embodiment, a terminal ofthe second flying capacitor Cb22 is electrically connected with thefirst terminal of the first winding T21, and the other terminal of thesecond flying capacitor Cb22 is electrically connected with the firstterminal of the second winding T23. The first terminal of the secondwinding T23 and the first terminal of the first winding T21 areopposite-polarity terminals.

In the second serially-connected branch, the positions and sequence ofthe second winding T24 and the first flying capacitor Cb21 are notrestricted. In an embodiment, the first terminal of the second windingT24 is electrically connected with the first terminal of the firstwinding T22. The first terminal of the second winding T24 and the firstterminal of the first winding T22 are opposite-polarity terminals. Thesecond terminal of the second winding T24 is electrically connected withthe first flying capacitor Cb21. In another embodiment, a terminal ofthe first flying capacitor Cb21 is electrically connected with the firstterminal of the first winding T22, and the other terminal of the firstflying capacitor Cb21 is electrically connected with the first terminalof the second winding T24. The first terminal of the second winding T24and the first terminal of the first winding T22 are opposite-polarityterminals.

The working principle of the power conversion circuit 2 will bedescribed as follows. For illustration, taking the first terminal of thepower conversion circuit 2 as the input terminal, and the secondterminal of the power conversion circuit 2 as the output terminal forexample.

Please refer to FIGS. 2B, 2C and 2D again. When the power conversioncircuit 2 is in a steady state, the time interval between the time pointt0 and the time point t4 is equal to the switching cycle Ts.

In the time interval between the time point t0 and the time point t1,the first switch S21, the second switch S24 and the sixth switch Sr21are in the on state. This time interval is also referred as a firstworking period. The first flying capacitor Cb21 is charged by the inputvoltage V1 through the first switch S21. The electric energy istransmitted from the input terminal to the output terminal through thesecond winding T24 and the first winding T22. The energy stored in thesecond flying capacitor Cb22 is transmitted to the output terminalthrough the second switch S24, the first winding T22, the sixth switchSr21 and the second winding T23. The first winding T21 is in afreewheeling state through the sixth switch Sr21. Meanwhile, the sum ofthe current flowing through the second winding T23 and the currentflowing through the second winding T24 is equal to the current flowingthrough the first winding T22. The equivalent circuit diagram is shownin FIG. 2C. In FIG. 2C, T21′, T22′, T23′ and T24′ are the ideal windingscorresponding to the windings T21, T22, T23 and T24, Lr21, Lr22, Lr23and Lr24 are equivalent leakage inductors corresponding to the windings,and Lm2 is an equivalent magnetized inductor of the magnetic elementT-2. Due to the resonant effect between the power conversion circuit 2(i.e., the equivalent resonant inductor resulted from the equivalentleakage inductors Lr21, Lr22, Lr23 and Lr24) and the flying capacitorsCb21 and Cb22, the resonant currents iLr21 and iLr22 are generated. Theequivalent magnetized current generated by the magnetic element T-2 isiLm2.

The associated voltages of the power conversion circuit 2 can be seen inFIG. 2C. The voltage between the two terminals of the ideal firstwinding T21′ is equal to the voltage V2 of the second terminal of thepower conversion circuit 2. As mentioned above, the turn ratio betweenthe second winding T23, the second winding T24, the first winding T21and the first winding T22 is N:N:1:1. Consequently, the voltage betweenthe two terminals of the ideal first winding T22′ is also equal to thevoltage V2, the voltage between the two terminals of the ideal secondwinding T23′ is equal to N×V2, and the voltage between the two terminalsof the ideal second winding T24′ is equal to N×V2.

Consequently, the voltage V1 of the first terminal of the powerconversion circuit 2 and the voltage Vc22 of the second flying capacitorCb22 may be expressed by the following mathematical formula:V1=Vc21+(2+N)×V2  (8); andVc22=(2+N)×V2  (9)

In the above mathematic formula, Vc21 is the terminal voltage of thefirst flying capacitor Cb21, and Vc22 is the terminal voltage of thesecond flying capacitor Cb22. At the time point t1, the resonantcurrents iLr21 and iLr22 are equal to the magnetized currents iLm1 and−iLm1, respectively. Meanwhile, the first switch S21, the second switchS24 and the sixth switch Sr21 are turned off. Since the zero currentswitching (ZCS) function is achieved, the switching loss is decreasedand the energy transfer efficiency of the power conversion circuit 2 isenhanced.

In the time interval between the time point t1 and the time point t2,all switches are turned off. The magnetized current iLm2 flowing throughthe magnetic element T-2 is in the freewheeling state. In addition, thecharges on the parasitic capacitors of the fourth switch S23, the fifthswitch S22 and the third switch Sr22 are extracted. Consequently, theterminal voltages of the fourth switch S23, the fifth switch S22 and thethird switch Sr22 are decreased. In an embodiment, the fourth switchS23, the fifth switch S22 and the third switch Sr22 are turned on whenthe terminal voltages of the fourth switch S23, the fifth switch S22 andthe third switch Sr22 are decreased to 50% of the respective initialvoltages (i.e., the terminal voltages at the time point t1).Consequently, the switching loss is decreased, and the energy transferefficiency and the power density of the power conversion circuit 2 areenhanced.

In another embodiment, the inductance of the magnetic element T-2 iscontrolled. Consequently, the inductance of the equivalent magnetizedinductor Lm2 of the magnetic element T-2 is low enough, and themagnetized current iLm2 flowing through the equivalent magnetizedinductor Lm2 is high enough. Since the charges on the parasiticcapacitors of the fourth switch S23, the fifth switch S22 and the thirdswitch Sr22 are extracted completely, the terminal voltages of thefourth switch S23, the fifth switch S22 and the third switch Sr22 aredecreased to zero. At this time, the fifth switch S22 and the thirdswitch Sr22 are turned on, the fourth switch S23, the fifth switch S22and the third switch Sr22 are turned on. Consequently, the zero voltageswitching (ZVS) function is achieved. In such way, the switching loss isfurther decreased, and the energy transfer efficiency and the powerdensity of the power conversion circuit 2 are further enhanced.

In the time interval between the time point t2 and the time point t3,the fourth switch S23, the fifth switch S22 and the third switch Sr22are in the on state. This time interval is also referred as a secondworking period. The voltage V1 of the input terminal is transmitted tothe second flying capacitor Cb22 through the fourth switch S23 so as tocharge the second flying capacitor Cb22. In addition, the energy storedin the second flying capacitor Cb22 is transmitted to the outputterminal through the second winding T23 and the first winding T21. Theenergy stored in the first flying capacitor Cb21 is transmitted to theoutput terminal through the fifth switch S22, the first winding T21, thethird switch Sr22 and the second winding T24. The first winding T22 isin the freewheeling state through the third switch Sr22. Meanwhile, thesum of the current flowing through the second winding T23 and thecurrent flowing through the second winding T24 is equal to the currentflowing through the first winding T21. The equivalent circuit diagram isshown in FIG. 2D. Due to the resonant effect between the powerconversion circuit 2 (i.e., the equivalent resonant inductor resultedfrom the equivalent leakage inductors Lr21, Lr22, Lr23 and Lr24) and theflying capacitors Cb21 and Cb22, the resonant currents iLr21 and iLr22are generated. The equivalent magnetized current generated by themagnetic element T-2 is iLm2.

The associated voltages of the power conversion circuit 2 can be seen inFIG. 2D. The voltage between the two terminals of the ideal firstwinding T22′ is equal to the voltage V2 of the second terminal of thepower conversion circuit 2. As mentioned above, the turn ratio betweenthe second winding T23, the second winding T24, the first winding T21and the first winding T22 is N:N:1:1. Consequently, the voltage betweenthe two terminals of the ideal first winding T21′ is also equal to thevoltage V2, the voltage between the two terminals of the ideal secondwinding T23′ is equal to N×V2, and the voltage between the two terminalsof the ideal second winding T24′ is equal to N×V2.

Consequently, the voltage V1 of the first terminal of the powerconversion circuit 2 and the voltage Vc21 of the first flying capacitorCb21 may be expressed by the following mathematical formula:V1=Vc22+(2+N)×V2  (10); andVc21=(2+N)×V2  (11)

The energy stored in the first flying capacitor Cb21 in the timeinterval between the time point t0 and the time point t1 is transmittedto the output terminal in the time interval between the time point t2and the time point t3. The energy stored in the second flying capacitorCb22 in the time interval between the time point t2 and the time pointt3 is transmitted to the output terminal in the time interval betweenthe time point t0 and the time point t1. According to the formulae (8),(9), (10) and (11), the voltage V1 of the first terminal of the powerconversion circuit 2 may be deduced as: V1=(4+2N)×V2.

At the time point t3, the resonant currents iLr21 and iLr22 are equal tothe magnetized currents iLm2 and -iLm2, respectively. Meanwhile, thefourth switch S23, the fifth switch S22 and the third switch Sr22 areturned off. Since the zero current switching (ZCS) function is achieved,the switching loss is decreased and the energy transfer efficiency ofthe power conversion circuit 2 is enhanced.

In the time interval between the time point t3 and the time point t4,all switches are turned off. The magnetized current iLm2 flowing throughthe magnetic element T-2 is in the freewheeling state. In addition, thecharges on the parasitic capacitors of the first switch S21, the secondswitch S24 and the sixth switch Sr21 are extracted. Consequently, theterminal voltages of the first switch S21, the second switch S24 and thesixth switch Sr21 are decreased. In an embodiment, the first switch S21,the second switch S24 and the sixth switch Sr21 are turned on when theterminal voltages of the first switch S21, the second switch S24 and thesixth switch Sr21 are decreased to 50% of the respective initialvoltages (i.e., the terminal voltages at the time point t1).Consequently, the switching loss is decreased, and the energy transferefficiency and the power density of the power conversion circuit 2 areenhanced.

In another embodiment, the inductance of the magnetic element T-2 iscontrolled. Consequently, the inductance of the equivalent magnetizedinductor Lm2 of the magnetic element T-2 is low enough, and themagnetized current iLm2 flowing through the equivalent magnetizedinductor Lm2 is high enough. Since the charges on the parasiticcapacitors of the first switch S21, the second switch S24 and the sixthswitch Sr21 are extracted completely, the terminal voltages of the firstswitch S21, the second switch S24 and the sixth switch Sr21 aredecreased to zero. At this time, the second switch S24 and the sixthswitch Sr21 are turned on, the first switch S21, the second switch S24and the sixth switch Sr21 are turned on. Consequently, the zero voltageswitching (ZVS) function is achieved. In such way, the switching loss isfurther decreased, and the energy transfer efficiency and the powerdensity of the power conversion circuit 2 are further enhanced.

In the time interval between the time point t0 and the time point t1 andin the time interval between the time point t2 and the time point t3,the resonant current iLr21 flows through the first winding T21 and theresonant current iLr22 flows through the first winding T22. In addition,the frequency of each of the resonant current iLr21 and the resonantcurrent iLr22 is equal to the switching frequency. In this embodiment,the resonant cycle and the switching cycle are nearly equal.

In some other embodiments, the capacitances of the flying capacitorsCb21 and Cb22 are larger, and the inductance of the equivalent resonantinductor is smaller. Consequently, if the resonant currents iLr22 andiLr21 are respectively greater than the magnetized currents iLm1 and−iLm1 in the time interval between the time point t0 and the time pointt1, the corresponding switches are turned off. If the resonant currentsiLr22 and iLr21 are respectively greater than the magnetized currents−iLm2 and iLm2 in the time interval between the time point t2 and thetime point t3, the corresponding switches are turned off. The turn-offcurrent is greater than zero. However, since the inductance of theequivalent resonant inductor is low, the power loss caused by thenon-zero current turning-off action may be neglected. In other words,the switching cycle of the power conversion circuit 2 is less than orequal to the resonant cycle of the resonant current. For reducing thepower loss and increasing the energy transfer efficiency, it ispreferred that the switching cycle Ts is greater than or equal to a halfof the resonant cycle.

In an embodiment, the ratio of the input voltage V1 to the outputvoltage V2 is (4+2N):1. That is, the ratio of the input voltage V1 tothe output voltage V2 may be adjusted according to the change of N. Inthis embodiment, the magnetic element T-2 includes the two firstwindings T21, T22 and the two second windings T23 and T24. Thesewindings interact with each other to result in the electromagneticcoupling effect. Moreover, the turn ratio between the second windingT23, the second winding T24, the first winding T21 and the first windingT22 is N:N:1:1. Since the voltage gain of the power conversion circuit 2is adjustable according to the turn numbers of the second winding T23and T24, the applications of the power conversion circuit 2 areexpanded.

In the embodiment, as shown in FIGS. 2C and 2D, the equivalent leakageinductors corresponding to the windings T21, T22, T23 and T24 are Lr21,Lr22, Lr23 and Lr24, respectively. For clearly analyzing therelationship between the resonant currents, the magnetized current iLm2and the magnetized voltage VLm2 of the equivalent magnetized inductorLm2 are neglected in the following example. In the time interval betweenthe time point t0 and the time point t1 and the time interval betweenthe time point t2 and the time point t3, the resonant effect between theflying capacitors Cb21, Cb22 and the equivalent leakage inductors Lr21,Lr22, Lr23 and Lr24 is generated. In this embodiment, the resonantcapacitance of the power conversion circuit 2 is the sum of thecapacitances of the flying capacitors Cb21 and Cb22, and the equivalentresonant inductance is equal to the parallel conductance of theequivalent leakage inductances Lr23 and Lr24 plus the equivalent leakageinductances Lr21 and Lr22 (i.e., the resonant capacitance of the powerconversion circuit 2 is Lr23∥Lr24+Lr21+Lr22). If the magnetized currentiLm2 is neglected, the output current io of the power conversion circuit2 may be expressed by the following mathematic formula:io=iLr21+iLr22  (12)

In the above mathematic formula, iLr21 is the resonant current flowingthrough the equivalent leakage inductor Lr21, and iLr22 is the resonantcurrent flowing through the equivalent leakage inductor Lr22.

In the time interval between the time point t0 and the time point t1,the resonant current iLr23 flowing through the equivalent leakageinductor Lr23 is equal to the resonant current iLr23 flowing through theequivalent leakage inductor Lr24. That is,iLr23=iLr24  (13)

The sum of the resonant current flowing through the equivalent leakageinductor Lr23 and the resonant current flowing through the equivalentleakage inductor Lr24 is equal to the resonant current iLr22 flowingthrough the equivalent leakage inductor Lr22. That is,iLr23+iLr24=iLr22  (14)

According to the magnetic potential balance principle, the followingmathematic formula is obtained.N×iLr23+N×iLr24+iLr22=iLr21  (15)

According to the above mathematic formulae (13), (14) and (15), theresonant current iLr12 is equal to io/(N+2), and the resonant currentiLr21 is equal to (N+1)×io/(N+2).

In the time interval between the time point t2 and the time point t3,the resonant current iLr23 flowing through the equivalent leakageinductor Lr23 is equal to the resonant current iLr24 flowing through theequivalent leakage inductor Lr24 (e.g., the formula (13)), and the sumof the resonant current flowing through the equivalent leakage inductorLr23 and the resonant current flowing through the equivalent leakageinductor Lr24 is equal to the resonant current iLr22 flowing through theequivalent leakage inductor Lr22 (e.g., the formula (14)).

According to the magnetic potential balance principle, the followingmathematic formula is obtained.N×iLr23+N×iLr24+iLr21=iLr22  (16)

According to the above mathematic formulae (13), (14) and (16), theresonant current iLr21 is equal to io/(N+2), and the resonant currentiLr22 is equal to (N+1)×io/(N+2).

From the above descriptions, the voltage gain of the power conversioncircuit 2 is adjustable according to the turn numbers of the secondwinding T23 and T24 of the magnetic element T-2. The sum of the resonantcurrents iLr21 and iLr22 in the time interval between the time point t0and the time point t1 and the sum of the resonant currents iLr21 andiLr22 in the time interval between the time point t2 and the time pointt3 are equal. In other words, the resonant effect of the powerconversion circuit 2 is not influenced by the second windings T23 andT24. The terminal voltage Vc21 of the first flying capacitor Cb21 isobtained by superimposing a DC voltage with an AC resonant voltage.Similarly, the terminal voltage Vc22 of the second flying capacitor Cb22is obtained by superimposing a DC voltage with an AC resonant voltage.Typically, the DC voltage is equal to Vin/2, and thus the ratio of theDC voltage to the input voltage (e.g., the voltage of the first terminalof the power conversion circuit 2) is 0.5. However, when the deviceparameter distribution and other factors are taken into consideration,the ratio of the DC voltage to the input voltage is in the range between0.4 and 0.6. The amplitudes of the AC resonant voltage of the terminalvoltages Vc21 and Vc22 are determined according to the inductance of theequivalent resonant inductor, the capacitance of the equivalent resonantcapacitor, the switching frequency of the power conversion circuit andthe size of the load.

In the above embodiment, the equivalent resonant inductor comprises theleakage inductance caused by the coupling effect of the windings T21,T22, T23 and T24 and the parasitic inductance of the wiring structure.When the resonant cycle, the switching cycle and the capacitances of theflying capacitors Cb21 and Cb22 are taken into consideration, thecoupling efficiency between every two of the windings T21, T22, T23 andT24 is preferably greater than 0.9.

FIG. 2E is a schematic circuit diagram illustrating a power conversioncircuit according to a fourth embodiment of the present invention. Incomparison with the third embodiment, the power conversion circuit ofthis embodiment further includes at least one external inductor (notshown).

The position of the at least one external inductor may be determinedaccording to the practical requirements. In an example, one externalinductor is serially connected between the second terminals of the firstwinding T21 or T22 and the second positive electrode V2+. That is, theexternal inductor is located at the position D. In another embodiment,two external inductors with the same inductance are serially connectedwith two first windings T21 and T22. That is, the two external inductorsare located at the positions E1 or E2. In an example, at least oneexternal inductor is serially connected to the first/secondserially-connected branch. For example, one external inductor is locatedat the position F1, F2 or F3 two external inductors are respectivelylocated at two of the positions F1, F2 or F3, or three externalinductors are respectively located at the positions F1, F2 or F3.Consequently, a suitable resonant cycle is acquired. Nevertheless, theat least one external inductor is serially connected between the firstterminal of the first switch S21 and the second positive electrode V2+.

In some embodiments, the third switch Sr22 and the sixth switch Sr21 arereplaced by diodes. The diodes are used as freewheeling diodes. Theswitching cycle of the power conversion circuit is less than or equal tothe resonant cycle. The equivalent circuit and the current waveform aresimilar to those of the above embodiment, and not redundantly describedherein. For example, the switches are controllable switches such as MOSswitches, SiC switches or GaN switches.

As mentioned above, the power conversion circuit 2 of the presentinvention has the function of converting the electric power in thebidirectional manner. Consequently, in case that the first terminal ofthe power conversion circuit 2 is the output terminal, the secondterminal of the power conversion circuit 2 is the input terminal. Theoperations are similar to those of the first embodiment, and are notredundantly described herein. Under this circumstance, the ratio of theinput voltage to the output voltage of the power conversion circuit 2 is1:(4+2N).

The present invention further provides a power conversion apparatus. Thepower conversion apparatus includes M power conversion circuits, M is aninteger greater than 1. The M power conversion circuits are connectedwith each other in an interleaving manner. Consequently, the carryingcapability of the power conversion system is enhanced. The firstterminals of the M power conversion circuits are connected with eachother. The second terminals of the M power conversion circuits areconnected with each other. The circuitry structures and the circuitryparameters of the M power conversion circuits are identical.

In an embodiment, each power conversion circuit has the circuit topologyas shown in FIG. 1A. In case that M is an odd value, the M powerconversion circuits are controlled according to M control signals. Eachpower conversion circuit is controlled according to one correspondingcontrol signal. The phase difference between the control signals forcontrolling every two adjacent power conversion circuits is in the rangebetween (360/M−20) degree and (360/M+20) degree. In case that M is aneven value, the M power conversion circuits are controlled according toM/2 control signals. Every two power conversion circuits are controlledaccording to one corresponding control signal. For example, the m-thpower conversion circuit and the (M/2+m)-th power conversion circuit arecontrolled according to the m-th control signal. The phase differencebetween the control signals for controlling every two adjacent powerconversion circuits is in the range between (360/M−20) degree and(360/M+20) degree, wherein m is an integer smaller than M.

In another embodiment, each power conversion circuit has the circuittopology as shown in FIG. 2A. The M power conversion circuits arecontrolled according to M control signals. Each power conversion circuitis controlled according to one corresponding control signal. The phasedifference between the control signals for controlling every twoadjacent power conversion circuits is in the range between (360/2M−20)degree and (360/2M+20) degree.

In the following embodiments, taking the power conversion apparatusincluding two power conversion circuits as an example to illustrate theembodiments, where the two power conversion circuits are connected witheach other in an interleaving manner.

FIG. 3 is a schematic circuit diagram illustrating a power conversionapparatus according to a first embodiment of the present invention. Asshown in FIG. 3 , the power conversion apparatus 100 includes two powerconversion circuits 1. Each power conversion circuit 1 has the circuitrystructure as shown in FIG. 1A. The first terminals of the two powerconversion circuits 1 are electrically connected with each other. Thesecond terminals of the two power conversion circuits 1 are electricallyconnected with each other.

In the embodiment as shown in FIG. 3 , each power conversion circuit 1of the power conversion apparatus 100 includes a first capacitor C1 anda second capacitor C2. In a variant example, one first capacitor C1 isshared by the first terminals of the power conversion circuits 1, andone second capacitor C2 is shared by the second terminals of the powerconversion circuits 1.

FIG. 4 is a schematic circuit diagram illustrating a power conversionapparatus according to a second embodiment of the present invention. Asshown in FIG. 4 , the power conversion apparatus 110 includes two powerconversion circuits 2. Each power conversion circuit 2 has the circuitrystructure as shown in FIG. 2A. The first terminals of the two powerconversion circuits 2 are electrically connected with each other. Thesecond terminals of the two power conversion circuits 2 are electricallyconnected with each other.

In the embodiment as shown in FIG. 4 , each power conversion circuit 2of the power conversion apparatus 110 includes a first capacitor C1 anda second capacitor C2. In a variant example, one first capacitor C1 isshared by the first terminals of the power conversion circuits 2, andone second capacitor C2 is shared by the second terminals of the powerconversion circuit 2.

From the above descriptions, the present invention provides a powerconversion circuit and a power conversion apparatus. The magneticelement of the power conversion circuit includes two first winding andat least one second winding. These windings interact with each other toresult in the electromagnetic coupling effect. The turn ratio betweenthe second winding and the first winding is N:1. Since the voltage gainof the power conversion circuit is adjustable according to the turnnumber of the second winding, the applications of the power conversioncircuit are expanded.

While the invention has been described in terms of what is presentlyconsidered to be the most practical and preferred embodiments, it is tobe understood that the invention needs not be limited to the disclosedembodiment. On the contrary, it is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the appended claims which are to be accorded with the broadestinterpretation so as to encompass all such modifications and similarstructures.

What is claimed is:
 1. A power conversion circuit, comprising: a firstterminal including a first positive electrode and a first negativeelectrode; a second terminal including a second positive electrode and asecond negative electrode, wherein the second negative electrode iselectrically connected with the first negative electrode; a firstswitching conversion unit comprising a first switch and a third switch,which are electrically connected with each other in series; a secondswitching conversion unit comprising a second switch and a fourthswitch, which are electrically connected with each other in series,wherein a first terminal of the first switch is electrically connectedwith a first terminal of the second switch, a second terminal of thefirst switch is electrically connected with the first positiveelectrode, a first terminal of the third switch and a first terminal ofthe fourth switch are electrically connected with the first negativeelectrode, and a second terminal of the fourth switch is electricallyconnected with a second terminal of the second switch, wherein the firstswitch, the second switch, the third switch and the fourth switch areperiodically operated at a switching cycle, and the switching cycle hasa duty cycle; a flying capacitor; and a magnetic element comprising twofirst windings and a second winding, wherein the two first windings andthe second winding interact with each other to result in anelectromagnetic coupling effect, wherein second terminals of the twofirst windings are opposite-polarity terminals and electricallyconnected with the second positive electrode, a first terminal of afirst one of the two first windings is electrically connected with asecond terminal of the third switch, a first terminal of a second one ofthe two first windings is electrically connected with the secondterminal of the fourth switch, and the second winding and the flyingcapacitor are serially connected between the first terminal of the firstswitch and the first terminal of the first one of the two firstwindings, wherein a turn ratio between the second winding, the first oneof the two first windings and the second one of the two first windingsis N:1:1, and N is a positive value.
 2. The power conversion circuitaccording to claim 1, wherein the switching cycle comprises a firstworking period and a second working period, wherein a current flowingthrough the second winding is equal to a current flowing through thefirst one of the two first windings during the first working period, andthe current flowing through the second winding is equal to a currentflowing through the second one of the two first windings during thesecond working period.
 3. The power conversion circuit according toclaim 1, wherein the power conversion circuit has a function ofconverting electric power in a bidirectional manner, wherein if thefirst terminal of the power conversion circuit is an input terminal, thesecond terminal of the power conversion circuit is an output terminal,wherein if the first terminal of the power conversion circuit is theoutput terminal, the second terminal of the power conversion circuit isthe input terminal.
 4. The power conversion circuit according to claim1, wherein a first terminal of the second winding and the first terminalof the first one of the two first windings are opposite-polarityterminals, the first terminal of the second winding is electricallyconnected with the first terminal of the first one of the two firstwindings, and a second terminal of the second winding is electricallyconnected with the flying capacitor; or wherein a first terminal of theflying capacitor is electrically connected with the first terminal ofthe first one of the two first windings, and a second terminal of theflying capacitor is electrically connected with the first terminal ofthe second winding.
 5. The power conversion circuit according to claim1, wherein on/off states of the first switch and the fourth switch areidentical, on/off states of the second switch and the third switch areidentical, and a phase difference between the on/off states of the firstswitch and the second switch is 180 degrees, wherein time durations ofthe first switch and the second switch are less than or equal to 0.5×Tsand greater than or equal to 0.4×Ts, wherein the switching period is Ts.6. The power conversion circuit according to claim 1, wherein the powerconversion circuit comprises an equivalent resonant inductor, and aresonant effect between the equivalent resonant inductor and the flyingcapacitor generates a resonant current with a resonant frequency and aresonant cycle, wherein the switching cycle of the power conversioncircuit is less than or equal to the resonant cycle, and the switchingcycle is greater than or equal to a half of the resonant cycle.
 7. Thepower conversion circuit according to claim 6, wherein the equivalentresonant inductor comprises a leakage inductance of the magnetic elementand a parasitic inductance of a wiring structure; or the equivalentresonant inductor comprises at least one external inductor, and the atleast one external inductor is serially connected between the firstterminal of the first switch and the second positive electrode.
 8. Thepower conversion circuit according to claim 1, wherein the two firstwindings and the second winding are wound around a same pillar of amagnetic core of the magnetic element, wherein a coupling efficiencybetween every two of the two first windings and the second winding isgreater than 0.9.
 9. The power conversion circuit according to claim 1,wherein during the switching cycle, electric energy is stored in theflying capacitor and transmitted to an output terminal of the powerconversion circuit according to the on/off states of the correspondingswitches of the first switch, the second switch, the third switch andthe fourth switch, wherein the flying capacitor has a DC voltage, thefirst terminal of the power conversion circuit has a terminal voltage,and a ratio of the DC voltage to the terminal voltage is in a rangebetween 0.4 and 0.6.
 10. The power conversion circuit according to claim1, wherein a ratio of a terminal voltage of the first terminal of thepower conversion circuit to a terminal voltage of the second terminal ofthe power conversion circuit is (4+2N):1.
 11. A power conversioncircuit, comprising: a first terminal including a first positiveelectrode and a first negative electrode; a second terminal including asecond positive electrode and a second negative electrode, wherein thesecond negative electrode is electrically connected with the firstnegative electrode; a magnetic element comprising two first windings andat least one second winding, wherein the two first windings and the atleast one second winding interact with each other to result in anelectromagnetic coupling effect, wherein second terminals of the twofirst windings are opposite-polarity terminals and electricallyconnected with the second positive electrode, wherein a turn ratiobetween the at least one second winding, a first one of the two firstwindings and a second one of the two first windings is N:1:1, and N is apositive value; a plurality of switches, wherein each of the pluralityof switches is electrically connected with the magnetic element, whereinthe plurality of switches are periodically operated at a switchingcycle, and the switching cycle has a duty cycle; wherein the switchingcycle comprises a first working period and a second working period,wherein a current flowing through the at least one second winding isequal to a current flowing through the first one of the two firstwindings during the first working period, and the at least one secondwinding is serially connected with the first one of the two firstwindings, wherein the current flowing through the at least one secondwinding is equal to a current flowing through the second one of the twofirst windings during the second working period, and the at least onesecond winding is serially connected with the second one of the twofirst windings.
 12. The power conversion circuit according to claim 11,wherein the power conversion circuit comprises a flying capacitor, andthe flying capacitor and the at least one second winding form aserially-connected branch, wherein the serially-connected branch iselectrically connected between one of the plurality of switches and thefirst one of the two first windings.
 13. The power conversion circuitaccording to claim 12, wherein the power conversion circuit comprises anequivalent resonant inductor, and a resonant effect between theequivalent resonant inductor and the flying capacitor generates aresonant current with a resonant frequency and a resonant cycle, whereinthe switching cycle of the power conversion circuit is less than orequal to the resonant cycle, and the switching cycle is greater than orequal to a half of the resonant cycle.
 14. The power conversion circuitaccording to claim 13, wherein the equivalent resonant inductorcomprises a leakage inductance of the magnetic element and a parasiticinductance of a wiring structure; or the equivalent resonant inductorcomprises at least one external inductor, and the at least one externalinductor is serially connected between one of the plurality of switchesand the second positive electrode.
 15. The power conversion circuitaccording to claim 12, wherein during the switching cycle, electricenergy is stored in the flying capacitor and transmitted to an outputterminal of the power conversion circuit according to the on/off statesof the corresponding switches, wherein the flying capacitor has a DCvoltage, the first terminal of the power conversion circuit has aterminal voltage, and a ratio of the DC voltage to the terminal voltageis in a range between 0.4 and 0.6.
 16. The power conversion circuitaccording to claim 11, wherein a ratio of a terminal voltage of thefirst terminal of the power conversion circuit to a terminal voltage ofthe second terminal of the power conversion circuit is (4+2N):1.
 17. Thepower conversion circuit according to claim 11, wherein a part of theplurality of switches are electrically connected with the first positiveelectrode of the power conversion circuit, and another part of theplurality of switches are electrically connected with the first negativeelectrode of the power conversion circuit.